Since the field effect transistor (FET) using gallium nitride (GaN), gallium arsenide (GaAs), or some other compound for semiconductors is capable of high output and has excellent high-frequency characteristics, it has been widely used in practice as the semiconductor device for use in the microwave spectrum.
For the conventional semiconductor device of this type, the flat planar active layer, including the flat planar drain region and the source region and the channel region between these, is usually formed on top of the compound semiconductor layer.
In such a semiconductor device, in order to accomplish higher output by applying a high voltage between the drain region and the source region, a high voltage resistance is required in the device.